Method of production of high-pressure seamless cylinder from corrosion-resistant steel

ABSTRACT

Blank of corrosion-resistant steel is coated with coating of water glass with thickness of 20 to 150 μm, cured at 15 to 60° C., and heated in an induction furnace to the temperature of 1180° C. to 1260° C. The heated up blank is without cooling by water descaling transferred into an extrusion press and extruded here, while the coating is broken to pieces and largely removed. Drawing in a horizontal drawing press and necking are performed. After forming to the final shape of the cylinder, leftovers of coating are removed by pressure blasting. A corrosion-resistant thin-walled seamless high-pressure cylinder with volume of 5 to 260 litres is manufactured.

TECHNICAL FIELD

The invention relates to the field of material forming, namely themethod of production of high-pressure cylinder from corrosion-resistantsteel using the method of hot backward extrusion solved completelyspecifically for the possibility to obtain high-pressure seamlesscylinder not showing hydrogen embrittlement to and inner surfacecorrosion.

BACKGROUND ART

High-pressure seamless steel cylinders are currently produced using themethod of backward extrusion and drawing utilizing the process accordingto the CZ pat. 243247.

When using the procedure according to the CZ patent 243247, steel blanksare first cut from billets of square or circular section. The steelblanks are heated in an induction furnace to the output temperature of1000 to 1250° C., after which they are robotically transposed into adescaling device where a high-pressure water jetting is performed toremove the scales from the surface of blanks. Then each individual blankis inserted by stacker into an extrusion press, where it is upset andbackward extruded. The backward extrusion process is realized in twosteps. During the first step, the blank is inserted into a die with avertically moving ram, cylindrical insert, and piercing mandrel fittedwith a piercing head, where a thick-walled hollow semi-product, which issmooth inside without protrusions or bumps, is produced by means ofextrusion from the blank. At the end of extrusion in the extrusionpress, bottom is pressed in the semi-product basically to the finalthickness, however the semi-product has larger diameter than the finalproduct. After completion of the backward extrusion in the extrusionpress, the semi-product is robotically removed from the extrusion press,turned by 90°, and seated in this position into the horizontal drawingpress, where the second step of forming, which is backward drawing, willtake place. During this second step, the semi-product is put on adrawing mandrel with the already final inner diameter of the cylinder,after which it is extruded through a stripper ring and roller cartridgesfitted with reduction rollers. The semi-product on the mandrel isrolled, in which process reshaping of the semi-product wall thickness byapproximately 25% to 85% will take place, and the semi-product willacquire the desired outer and inner diameter. In addition, the rest ofscales will fall off. After the semi-product passing through thestripper ring and the roller cartridges, insertion of the semi-productbottom into a sizing die is performed, thus forming the final shape ofthe cylinder bottom. Then, in the course of backward movement of thedrawing mandrel, the semi-product is pulled off from the drawing mandrelwith the help of stripping jaws. The stage of the semi-productprocessing by rolling is mentioned in the Utility model CZ U 20492.After final forming by rolling, the hollow semi-products are aftercooledwith air and subsequently necked, i.e. closed by means of rotary formingto create a steel cylinder of typical geometry.

Disadvantage of the existing procedures of production of high-pressureseamless cylinders is the fact that they do not allow production ofcylinders from corrosion-resistant steels using the method of backwardextrusion and drawing, because the corrosion-resistant material degradeswhen conventional method of backward extrusion and drawing is used andit is impossible to produce safe high-pressure seamless cylinder fromit.

Currently there are also procedures that allow producing high-pressurecylinders from corrosion-resistant pipes. These are procedures in whichnecking of pipe segment is performed on both sides. In a seamless form,however, only double-necked cylinders of a simple shape can bemanufactured with these procedures, because they do not allow seamlessclosure of the bottom. Cylinders of different types of alloys such aschrome-molybdenum can also be produced using these procedures. However,because it is not possible to produce sufficiently thin-walled tubes asthe starting semi-product for these procedures, all such pressurebottles made using production methods based on pipes have thick-walledstructure, and thus they have considerable disadvantage in a very bigmass. Especially in the case of bigger and big volumes, such big weightof cylinders complicates handling, storage, and transportation. Existingthick-walled cylinders have another disadvantage in low strength.

In the steel industry, the use of water glass is known in the field offoundry practice for protection of casting moulds against damage. Inthis field, the inner surface of the casting mould is sometimes providedwith a coating of water glass so that this coating could prevent directcontact of molten metal with the mould surface and thereby increase theservice life of the mould. This coating is resistant to hightemperatures used in the foundry practice. Water glass is used in thefoundry industry also as a cementing component for the production offoundry moulds of sand.

DISCLOSURE OF INVENTION

The above-mentioned disadvantages are eliminated by the invention. Newmethod of manufacturing seamless high-pressure cylinder fromcorrosion-resistant steel is designed allowing the production ofthin-walled seamless version of the cylinder from corrosion-resistantsteel even for larger and high-volume high-pressure cylinders.

The invention starts from the existing method of manufacturing seamlesshigh-pressure cylinders. The starting part in the form of a steel blankis heated in an induction furnace and then inserted into an extrusionpress where it is upset and backward extruded in two steps. From this,during the first step, the blank is inserted into a die with acylindrical insert and a vertically moving piercing tool, and here it ismoulded by means of extrusion until a thick-walled hollow semi-producthaving an inner cavity, walls and bottom is moulded from it. After that,the semi-product is removed from the extrusion press, turned by 90°, andseated in this position into a horizontal drawing press, where thesecond step of forming, which is backward drawing, takes place. Duringthis second step, the semi-product is put on a drawing mandrel with thediameter corresponding to the desired final inner diameter of theproduced cylinder, while in this drawing press, the semi-product isextruded through a stripper ring and roller cartridges fitted withreduction rollers by means of which it is rolled on the mandrel. Thisrolling is performed so long until the semi-product acquires the desiredouter and inner diameter corresponding to the required final dimensionsof the produced cylinder. Then the final shape of the cylinder bottom isformed by insertion of the semi-product bottom into a sizing die andsubsequently, in the course of the backward movement of the drawingmandrel, the semi-product is pulled off from the drawing mandrel withthe help of stripping jaws and is aftercooled. Finally, the semi-productmanufactured in this way is necked, by which the shape of themanufactured cylinder is finished. The essence of the new solutionaccording to the invention is that even before heating in the inductionfurnace, at least 85% of the blank surface is coated with coating frommaterial based on water glass with thickness of 20 to 150 μm, thiscoating is cured by drying at the temperature of 15 to 60° C. and onlyafter this curing, the blank is subjected to heating in the inductionfurnace.

The material based on water glass is applied to the blank for example byspraying with nozzles or brushing in the form of water glass suspension.The water glass suspension stands here for suspension containing 20 to40% w. w. of sodium silicate or potassium silicate or mixture of thesesilicates, and 80 to 40% w. w. of water, while in the case of presenceof admixtures such as borosilicates and/or corrosion inhibitors, theseadmixtures are contained in the amount of maximum 20% w. w.

The blank with coating from material based on water glass is heated inthe induction furnace to the temperature of 1180° C. to 1260° C.

Then, when the blank with coating is heated up in the induction furnace,the blank with coating is taken out of here, and during persistence ofits temperature of minimum 1110° C., it is inserted into the extrusionpress, preferably without performing water descaling between the removalfrom the induction furnace and insertion into the extrusion press.

During the first step of mechanical working, in the course of piercing,the coating of material based on water glass is broken to pieces bypurposeful pushing of the piercing tool to the blank, while it iscracking off during extrusion in the extrusion press until at least itsmajor part is removed, meant as thickness of layer.

Then, when the final shape of the cylinder is formed from thesemi-product, the leftovers of coating based on water glass arepreferably removed from the surface of the semi-product by pressureblasting of its outer as well as inner surface with abrasive.

The blank is made preferably from corrosion-resistant steel while theresulting cylinder is made as a seamless cylinder in the volume rangefrom 5 litres up to 260 litres, as a single necked or double-neckedcylinder for any volume within the said range.

Thin walled seamless high-pressure cylinders of the abovementionedvolumes are preferably manufactured with the help of this invention. Inthe second step of forming, the semi-product wall is pressed to thethickness of 2 to 21.5 mm.

The invention is utilizable for manufacturing of seamless high-pressurecylinders. It makes possible to manufacture these cylinders fromcorrosion-resistant steel and that in the corrosion-resistant version,even for larger and high cylinder volumes from 5 to 260 litres. Theinvention makes possible to manufacture these cylinders in thethin-walled version from significantly wider range of high-strengthcorrosion-resistant steels than the existing methods. Using thisinvention, substantial reduction of the mass of high-pressure cylindersis achieved as compared to the present state of art, and savings ofmaterial for their production. The seamless thin-walled version withthese volumes was not possible so far. The cylinders according to theinvention exhibit high mechanical resistance as well as pressureresistance. They have relatively very low mass, which as compared to thepresent state of art facilitates manipulation, storage, as well astransport. As compared to cylinders currently manufactured for examplefor scuba diving, respirators, for calibration gases etc., the cylindersaccording to the invention are by as much as two thirds lighter. Theoption to use corrosion-resistant steel for the production makespossible to use these cylinders even for raw natural gas and for gasesand mixtures that in the case of existing cylinders from commonchrome-molybdenum steel are causing hydrogen embrittlement andaccelerated corrosion when reacting with the gas under high pressure.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated using drawings, where

FIG. 1 shows the blank with applied coating of water glass incross-section,

FIG. 2 shows the phase of curing of the coating on the blank,

FIG. 3 shows the phase of relocation of the blank from the inductionfurnace directly into the extrusion press, and

FIG. 4 A, B shows process of pressing of the blank in the extrusionpress, from which A shows the phase of extrusion of the future cylindercavity in the blank and B shows the subsequent phase of breaking andfalling off of the coating from the semi-product during pressing.

BEST MODE FOR CARRYING OUT THE INVENTION

Example of embodiment of the invention is visually demonstrated with thehelp of FIGS. 1 to 4 and the hereinafter-described method of productionof corrosion-resistant high-pressure seamless cylinder for storage,transport, and use of natural gas.

Parts of the size needed for the manufacture of cylinders of theproduced volume are first cut from billets of corrosion-resistant steelhaving square or circular section. Each individual part, i.e. thestarting steel blank 1, is on at least 85% of its surface coated withcoating 2 from material based on water glass. The thickness of appliedlayer is 20 to 150 μm. Suspension usually called water glass is used asthe material based on water glass. For the purposes of the invention,the suspension based on water glass stands for suspension containing 20to 40% w. w. of sodium silicate or potassium silicate or their mixtureand 80 to 40% w. w. of water. Admixtures of borosilicates and corrosioninhibitors such as hexamine, fenylethylamine, phosphates, etc. and/orpossible other admixtures can be contained provided they do not exceedthe amount of 20% w. w. in the suspension.

Examples of material composition of the suspension are given below.

Suspension I weight % in Material the suspension Silicates (sodium orpotassium silicate or their mixture) 20 Water 80 Admixtures 0

Suspension II weight % in Material the suspension Silicates (sodium orpotassium silicate or their mixture) 40 Water 60 Admixtures 0

Suspension III weight % in Material the suspension Silicates (sodium orpotassium silicate or their mixture) 30 Water 60 Admixtures(borosilicates and corrosion inhibitors 10 in quantities 1:1)

Suspension IV weight % in Material the suspension Silicates (sodium orpotassium silicate or their mixture) 40 Water 40 Admixtures(borosilicates and unidentified admixtures) 20

This coating 2 can be applied by brushing or spraying. After theapplication, the coating is let to dry at the temperature of 15 to 60°C. until curing. Sufficient curing will not take place in the case ofcuring below the temperature of 15° C., so that unwanted cracking andfalling-off of the coating 2 layer would take place during subsequentheating in the induction furnace. In the case of curing above 60° C.,unwanted cracking of the coating 2 will take place as soon as duringdrying. During curing within the said range, creation of homogenouslayer acting as a protective jacket on the surface of the blank 1 takesplace. For the drying process or after it, the blank 1 is placed intothe induction furnace, where it is heated up by gradual heating to thetemperature of 1180° C. to 1260° C. During heating, neitherhigh-temperature oxidation of the surface of blanks 1 contained in theinduction furnace nor forge welding of more pieces of blanks 1, imminentin the case of heating without the coating 2, takes place. The heated upblank 1 with the coating 2 is removed from the induction furnace byrobotic feeder 3 and immediately after removal from the inductionfurnace it is inserted into a die 4 of extrusion press withoutsignificant cooling down, with continuation of its temperature of atleast 1110° C.

When compared with the process used until now, the step of descaling bywater jetting of scale, which is always performed during the existingprocess between the removal from the induction furnace and insertioninto the extrusion press, is skipped. Skipping of the up to nownecessary step of water jetting of scale is very significant, becausethis is preventing cooling down by more than 80° C., thus eliminatingformation of temperature gradient and temperature fluctuations that aretypical during the clearing of high-temperature-heating generated scaleby means of high-pressure water jet descaling. In addition, thepossibility of precise control of the blank 1 temperature is achieved byskipping the descaling.

In the extrusion press, the blank 1 and later the semi-product 5 createdfrom it is upset and backward extruded in two steps. From this, duringthe first step, the blank 1 is placed at the bottom 6 of the extrusionpress die 4 and here it is pressed by vertically moving ram ended with apiercing mandrel constituting the piercing tool 7. The blank 1 ismoulded here by means of extrusion until a thick-walled hollowsemi-product 5 having an inner cavity, walls, and bottom is moulded fromit, as shown on FIGS. 4 A, B. During this first step of mechanicalworking, in the course of piercing, the coating 2 of material based onwater glass is broken to pieces by pushing of the piercing tool 7 to theblank 1 and the semi-product 5 made of it, the coating 2 cracks off, andapproximately all this coating 2 is removed during extrusion in theextrusion press. During extrusion, the water glass admixtures will maketheir way from the coating 2 into the surface layer of the semi-product5 to the depth of maximum 10 μm, these leftovers are removed later byblasting. The aforementioned extrusion takes place during continuationof the semi-product 5 temperature at 1100 to 1200° C. Homogenousmoulding of the first semi-product 5 does not take place and transversalcracks appear on the semi-product 5 body at a lower temperature. On thecontrary, when exceeding the temperature above 1200° C., oxidation ofprimary austenitic grains occurs and so called burning out of thematerial takes place and hence its irreversible degradation. Withoutusing the coating 2, the semi-product 5 could not be extruded and drawnto the required shape, or the corrosion-resistant steel could not beused as the material and/or the result could not lead to the productionof the thin-walled seamless high-pressure cylinder of requiredmechanical and pressure strength.

After completion of processing in the extrusion press, the semi-product5 is removed from the extrusion press, turned by 90°, and seated in thisposition into a horizontal drawing press, where the second step offorming in terms of backward drawing takes place. During this secondstep, the semi-product 5 is put on a drawing mandrel having the diametercorresponding to the desired final inner diameter of the producedcylinder, and in this drawing press, the semi-product 5 is extrudedthrough a stripper ring and roller cartridges fitted with reductionrollers by means of which it is rolled on the mandrel. Rolling isperformed so long until the semi-product 5 acquires the required outerand inner diameter. Then the final shape of the cylinder bottom isformed by insertion of the semi-product 5 bottom into a sizing die andsubsequently, in the course of the backward movement of the drawingmandrel, the semi-product 5 is pulled off from the drawing mandrel withthe help of stripping jaws, aftercooled, and subsequently necked byalready known method by means of rotary forming or die forging process.The shape of the manufactured cylinder is finished in this way.

Leftovers of the water glass based coating 2 are removed from thesurface of the semi-product 5 by pressure blasting of its outer as wellas inner surface with abrasive for example of steel-shot and cast-irongrit.

Corrosion-resistant seamless pressure cylinders in the volume range from5 litres up to 260 litres can be manufactured using the method accordingto the invention for any volume within the said range as single neckedor double-necked cylinders. In the second step of forming, the inventionallows pressing the semi-product 5 wall even to the thickness of athin-walled cylinder without loss of qualitative properties of themanufactured cylinder. The semi-product 5 wall is pressed even to thethickness of 2 to 21.5 mm. In so doing, the cylinder wall thickness isselected in concrete terms within the above said range depending on therequired cylinder volume, pressure for which the cylinder is designated,and requirements for qualitative properties and mechanical strength ofthe cylinder.

Figures FIG. 1 to 4 illustrate only the steps related directly to thenew steps of the process of production of the cylinder according to theinvention. The other steps and processes that are already known in thisfield and are described in the Background Art paragraph do not need tobe illuminated using figures.

1. Method of production of high-pressure seamless cylinder fromcorrosion-resistant steel, in which the starting steel blank (1) isheated in an induction furnace and then inserted into an extrusion presswhere it is upset and backward extruded in two steps, from which duringthe first step, the blank (1) is inserted into a die (4) with avertically moving piercing tool (7), and here it is moulded by means ofextrusion until a thick-walled hollow semi-product (5) having an innercavity, walls and bottom is moulded from it, after which thissemi-product (5) is removed from the extrusion press, turned by 90°, andseated in this position into a horizontal drawing press, where thesecond step of forming in terms of backward drawing takes place, whenduring this second step, the semi-product (5) is put on a drawingmandrel having the diameter corresponding to the required final innerdiameter of the produced cylinder, and in this drawing press, thesemi-product (5) is extruded through a stripper ring and rollercartridges fitted with reduction rollers by means of which it is rolledon the mandrel, while this rolling is performed so long until thesemi-product (5) acquires the required outer and inner diameter, afterwhich the final shape of the cylinder bottom is formed by insertion ofthe semi-product (5) bottom into a sizing die and subsequently, in thecourse of the backward movement of the drawing mandrel, the semi-product(5) is pulled off from the drawing mandrel with the help of strippingjaws, aftercooled, and subsequently necked, by which the shape of themanufactured cylinder is finished, characterized by that even beforeheating in the induction furnace, the blank (1) is coated on at least85% of its surface with coating (2) from material based on water glasswith thickness of 20 to 150μτπ, this coating (2) is cured by drying atthe temperature of 15 to 60° C. and only after this curing, the blank(1) is subjected to heating in the induction furnace.
 2. Method ofproduction of high-pressure seamless cylinder from corrosion-resistantsteel according to claim 1, characterized by that the material appliedto the blank (1) for example by spraying or by brush consists ofsuspension containing 20 to 40% w. w. of sodium silicate or potassiumsilicate or mixture of these silicates, and 40 to 80% w. w. of water,while in the case of presence of admixtures such as bore-silicatesand/or corrosion inhibitors, the admixtures are contained in the amountof maximum 20% w. w.
 3. Method of production of high-pressure seamlesscylinder from corrosion-resistant steel according to claim 1characterized by that the blank (1) with the coating (2) from materialbased on water glass is heated in the induction furnace to thetemperature of 1180° C. to 1260° C.
 4. Method of production ofhigh-pressure seamless cylinder from corrosion-resistant steel accordingto claim 1, characterized by that after the blank (1) with coating (2)is heated up in the induction furnace, the blank (1) with coating (2) istaken out of here and during persistence of its temperature of minimum1110° C., it is inserted into the extrusion press without performingwater jetting of scale between the removal from the induction furnaceand insertion into the extrusion press.
 5. Method of production ofhigh-pressure seamless cylinder from corrosion-resistant steel accordingto claim 1, characterized by that during the first step of mechanicalworking, in the course of piercing, the coating (2) of material based onwater glass is broken to pieces by pushing of the piercing tool (7) tothe blank (1) and the coating (2) cracks off during extrusion in theextrusion press until at least its predominant part is removed, 6.Method of production of high-pressure seamless cylinder fromcorrosion-resistant steel according to claim 5, characterized by thatafter the final shape of the cylinder is formed from the semi-product(5), the leftovers of coating (2) based on water glass are removed fromthe surface of the semi-product (5) by pressure blasting of its outer aswell as inner surface.
 7. Method of production of high-pressure seamlesscylinder from corrosion-resistant steel according to claim 1,characterized by that the blank (1) is made of corrosion-resistant steelwhile the resulting cylinder is made as a seamless cylinder in thevolume range from 5 litres up to 260 litres, as a single necked ordouble-necked cylinder for any volume within the said range.
 8. Methodof production of high-pressure seamless cylinder fromcorrosion-resistant steel according to claim 7, characterized by that inthe second step of forming, the semi-product (5) wall is pressed to thethickness of 2 to 21.5 mm.